In the related art, there is a known observation apparatus for observing a living sample, such as a small laboratory animal, with which observation is performed by disposing an objective lens in various directions or at various positions with respect to the sample (for example, see PTL 1).
In this observation apparatus disclosed in PTL 1, fluorescence generated at a light-focusing position due to a multiphoton excitation effect by focusing ultrashort pulsed laser light on a specimen is split off from the optical path of the ultrashort pulsed laser light immediately behind the objective lens, is guided to an external photodetector by an optical fiber, and is detected. By using a multiphoton-excitation-type observation apparatus like this, it is possible to focus the ultrashort pulsed laser light at a portion deep inside the sample, thus generating fluorescence, whereby it is possible to acquire a fluorescence image of the portion deep inside the sample.
In the case of the multiphoton-excitation-type observation apparatus, the fluorescence collected by the objective lens is split off immediately behind the objective lens, before returning to the scanner. Hence, when ultrashort pulsed laser light is scanned over the sample by the scanner, the position of a fluorescence beam also varies accordingly. Hence, in the case in which the scanning area scanned by the scanner is large, it is difficult to make the split-off fluorescence enter an optical fiber.
Furthermore, when the sample is a scattering material, such as a biological tissue, the fluorescence generated at the light-focusing position of the ultrashort pulsed laser light is scattered by the sample in the course of returning to the objective lens. In order to obtain a bright fluorescence image, it is necessary that fluorescence collected from a wide area of the sample by using the objective lens is detected.